There's a few ways to present a hookbait...
For hard baits you can superglue them onto the hook, superglue them onto a hair, attach them directly to the hook with a bait band (use silicone or latex rubber bands or floss), mount them by piercing the band with the point of the hook (prevents bait roll), you can also band some soft hookers too providing the band isn't too tight and holds it on without cutting it in two. You can also hair rig baits both hard and soft, you can lasso them, hair rig a band and use that to attach pellets, use pre-drilled hard baits and a stop with a hair or create a hole in them with a nut drill (basically any small drill bit). Other times you can hook baits directly if they're soft and firm - soft hookers are perfect for this as are worms, maggots and corn.
Its simply a case of presentation as others have said. Knowing what to do and when in a situation comes with knowing your target and what your likely to catch. Generally you would directly hook soft baits onto smaller hooks for small silver fish, hard band harder baits for chub on the river and hair rig for barbel and carp. The length of the hair is arbitrary these days as they can be made adjustable using a
splitstop so you could for example; use a very short hair for chub and adjust the length if you suspect barbel and carp are present.
As long as the hook size used is suitable for the size of fish you intend to catch, the bait can be as big, bigger or smaller than the hook when using a hair... you no longer need to exactly match the bait to the hook size when using hairs. that's not to say that a single piece of sweet corn on a size 4 intended for double figure carp would be better than a bigger mouthful... its all a question of balance.